Devices having horizontally-disposed nanofabric articles and methods of making the same

ABSTRACT

New devices having horizontally-disposed nanofabric articles and methods of making same are described. A discrete electro-mechanical device includes a structure having an electrically-conductive trace. A defined patch of nanotube fabric is disposed in spaced relation to the trace; and the defined patch of nanotube fabric is electromechanically deflectable between a first and second state. In the first state, the nanotube article is in spaced relation relative to the trace, and in the second state the nanotube article is in contact with the trace. A low resistance signal path is in electrical communication with the defined patch of nanofabric. Under certain embodiments, the structure includes a defined gap into which the electrically conductive trace is disposed. The defined gap has a defined width, and the defined patch of nanotube fabric spans the gap and has a longitudinal extent that is slightly longer than the defined width of the gap.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority under 35U.S.C. §120 to U.S. patent application Ser. No. 11/193,795, filed onJul. 29, 2005, entitled Devices Having Horizontally-Disposed NanofabricArticles and Methods of Making the Same, the entire contents of whichare incorporated herein by reference, which is a continuation of andclaims priority under 35 U.S.C. §120 to U.S. patent application Ser. No.10/776,059, filed on Feb. 11, 2004, entitled Devices HavingHorizontally-Disposed Nanofabric Articles and Methods of Making the Samewhich claims priority under 35 U.S.C. §119(e) to the followingapplications which are incorporated herein by reference in theirentirety: U.S. Provisional Patent Application No. 60/446,786, filed onFeb. 12, 2003, entitled Electro-Mechanical Switches and Memory CellsUsing Vertically-Disposed Nanofabric Articles and Methods of Making theSame, and U.S. Provisional Patent Application No. 60/446,783, filed onFeb. 12, 2003, entitled Electro-Mechanical Switches and Memory CellsUsing Horizontally-Disposed Nanofabric Articles and Methods of Makingthe Same; and which is a continuation-in-part and claims priority under35 U.S.C. §120 to the following applications which are incorporatedherein by reference in their entirety:

U.S. patent application Ser. No. 09/915,093, filed on Jul. 25, 2001, nowU.S. Pat. No. 6,919,592, entitled Electromechanical Memory Array UsingNanotube Ribbons and Method for Making Same;

U.S. patent application Ser. No. 10/033,323, filed on Dec. 28, 2001, nowU.S. Pat. No. 6,911,682, entitled Electromechanical Three-Trace JunctionDevices;

U.S. patent application Ser. No. 10/128,118, filed Apr. 23, 2002, nowU.S. Pat. No. 6,706,402, entitled Nanotube Films and Articles; and

U.S. patent application Ser. No. 10/341,005, filed on Jan. 13, 2003,entitled Methods of Making Carbon Nanotube Films, Layers, Fabrics,Ribbons, Elements and Articles.

TECHNICAL FIELD

The present application relates to devices having horizontally-disposednanofabric articles and to methods of making the same.

BACKGROUND

Memory devices have been proposed which use nanoscopic wires, such assingle-walled carbon nanotubes, to form crossbar junctions to serve asmemory cells. (ee WO 01/03208, Nanoscopic Wire-Based Devices, Arrays,and Methods of Their Manufacture; and Thomas Rueckes et al., “CarbonNanotube-Based Nonvolatile Random Access Memory for MolecularComputing,” Science, vol. 289, pp. 94-97, 7 Jul., 2000.) Hereinafterthese devices are called nanotube wire crossbar memories (NTWCMs). Underthese proposals, individual single-walled nanotube wires suspended overother wires define memory cells. Electrical signals are written to oneor both wires to cause them to physically attract or repel relative toone another. Each physical state (i.e., attracted or repelled wires)corresponds to an electrical state. Repelled wires are an open circuitjunction. Attracted wires are a closed state forming a rectifiedjunction. When electrical power is removed from the junction, the wiresretain their physical (and thus electrical) state thereby forming anon-volatile memory cell.

The NTWCM proposals rely on directed growth or chemical self-assemblytechniques to grow the individual nanotubes needed for the memory cells.These techniques are now believed to be difficult to employ atcommercial scales using modern technology. Moreover, they may containinherent limitations such as the length of the nanotubes that may begrown reliably using these techniques, and it may difficult to controlthe statistical variance of geometries of nanotube wires so grown.Improved memory cell designs are thus desired.

U.S. Pat. No. 6,919,592 discloses, among other things, electromechanicalcircuits, such as memory cells, in which circuits include a structurehaving electrically conductive traces and supports extending from asurface of a substrate. Nanotube ribbons are suspended by the supportsthat cross the electrically conductive traces. Each ribbon comprises oneor more nanotubes. The ribbons are formed from selectively removingmaterial from a layer or matted fabric of nanotubes.

For example, as disclosed in U.S. Pat. No. 6,919,592, a nanofabric maybe patterned into ribbons, and the ribbons can be used as a component tocreate non-volatile electromechanical memory cells. The ribbon iselectromechanically-deflectable in response to electrical stimulus ofcontrol traces and/or the ribbon. The deflected, physical state of theribbon may be made to represent a corresponding information state. Thedeflected, physical state has non-volatile properties, meaning theribbon retains its physical (and therefore informational) state even ifpower to the memory cell is removed. As explained in U.S. Pat. No.6,911,682, three-trace architectures may be used for electromechanicalmemory cells, in which the two of the traces are electrodes to controlthe deflection of the ribbon.

SUMMARY

The present invention provides new devices having horizontally-disposednanofabric articles and methods of making same.

Under certain aspects of the invention, a discrete electro-mechanicaldevice includes a structure having an electrically-conductive trace. Adefined patch of nanotube fabric is disposed in spaced relation to thetrace; and the defined patch of nanotube fabric is electromechanicallydeflectable between a first and second state. In the first state, thenanotube article is in spaced relation relative to the trace, and in thesecond state the nanotube article is in contact with the trace. A lowresistance signal path is in electrical communication with the definedpatch of nanofabric.

Under another aspect of the invention, the structure includes a definedgap into which the electrically conductive trace is disposed. Thedefined gap has a defined width, and the defined patch of nanotubefabric spans the gap and has a longitudinal extent that is slightlylonger than the defined width of the gap.

Under another aspect of the invention, the device includes anotherelectrically conductive trace in spaced relation the patch of nanotubefabric.

Under another aspect of the invention, a clamp is disposed at each oftwo ends of the nanotube fabric segment and disposed over at least aportion of the nanotube fabric segment substantially at the edgesdefining the gap.

Under another aspect of the invention, the clamp is made ofelectrically-conductive material.

Under another aspect of the invention, the clamp is made ofelectrically-insulative material having a via therethrough filled withelectrically conductive material to provide an electrical communicationpath with the nanotube fabric segment.

Under another aspect of the invention, the nanotube fabric segment ismade of a nanofabric having a porosity and wherein the electricallyconductive material filling the via also fills at least some of thepores of the of the nanotube fabric segment.

Under another aspect of the invention, the nanotube fabric segment has alithographically-defined shape.

Under another aspect of the invention, the contact between the nanotubepatch and the trace is a non-volatile state.

Under another aspect of the invention, the contact between the nanotubepatch and the trace is a volatile state.

Under another aspect of the invention, the at least one electricallyconductive trace has an interface material to alter the attractive forcebetween the nanotube fabric segment and the electrically conductivetrace.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing,

FIGS. 1A-P and 1I′-1K′ are cross-sectional diagrams that illustrateintermediate structures created during the process of forming a devicehaving a horizontally disposed nanotube article according to certainembodiments of the invention;

FIGS. 2A-D are cross-sectional diagrams that illustrate certainembodiments of the invention in which the gap displacement between asuspended nanotube article and an electrode may be controlled duringfabrication and also illustrate a metallization scheme according tocertain embodiments of the invention;

FIG. 3 illustrates a plan view of an intermediate structure according tocertain embodiments of the invention;

FIGS. 4-6 are perspective views of an intermediate structure shown invarious cross sectional views according to certain embodiments of theinvention;

FIG. 7 is a micrograph of an intermediate structure according to certainembodiments of the invention;

FIGS. 8A-B and 9 illustrate ways to strap or clamp articles made fromlayers of matted nanotubes with materials, including relatively lowresistance materials, according to certain embodiments of the invention;and

FIG. 10 is an image of an exemplary nanofabric shown in perspective.

DETAILED DESCRIPTION

Preferred embodiments of the invention provide new articles havinghorizontally-disposed nanotube articles and provide methods of makingsame. Some embodiments provide improved ways of clamping or pinchingsuspended nanotube articles to improve their performance andmanufacturability. Other embodiments provide electromechanical memorycells, which may be discrete or embedded. Under some embodiments, thediscrete memory cells use new approaches to connect to other circuitryor cells, which lowers the resistivity of traces to the memory cells.Still other embodiments provide memory cells that have volatileinformation state (i.e., the information state is lost when power isinterrupted). Some other embodiments use three-trace architecturesanalogous to those of U.S. Pat. No. 6,911,682. These embodiments howevermay utilize a combination of volatile and non-volatile characteristics;for example, information state may be non-volatile, but the device mayuse a three-trace architecture in which the deflection of the nanotubearticle may be caused by a trace having volatile state characteristics.

The preferred embodiments are made using nanotube films, layers, ornon-woven fabrics so that they form, or may be made to form, varioususeful patterned components, elements or articles. (Hereinafter “films,”“layers,” or “non-woven fabrics” are referred to as “fabrics” or“nanofabrics”.) The components created from the nanofabrics retaindesirable physical properties of the nanotubes and/or the nanofabricsfrom which they are formed. In addition, preferred embodiments allowmodern manufacturing techniques (e.g., those used in semiconductormanufacture) to be employed to utilize the nanofabric articles anddevices.

Preferred embodiments of the present invention include articles andmethods that increase a strain in the nanofabrics, allowing selectableconstruction of volatile and non-volatile electro-mechanical switches,including tri-state or tri-trace switches having both volatile andnon-volatile states. The nanofabrics in some embodiments also providefor discrete cellular articles, such as memory cells, to bemanufactured.

Briefly, FIGS. 2A-D illustrate discrete devices that have a nanotubearticle that is suspended relative to two control electrodes. The gapdistance between electrode and nanotube article may be controlled duringfabrication to result in different behavior of the device. Theseembodiments, among others, are discussed in more detail below.

FIGS. 3-6 show various plan and cross-section views of a device toillustrate the crossed- and spaced-relationship between the controlelectrodes and the nanotube article for a given cell or device.

Preferably, the nanotube patch or segment is clamped (above and below)up to the portion of the nanofabric article that is so suspended. Inaddition, preferably, the nanofabric article is connected or joined tohigh conductivity signal paths.

FIGS. 1A-P illustrate how individual, discrete devices or cells havingnanofabric articles may be made according to preferred embodiments ofthe invention. (The figures are not to scale.) The individual cellincludes a patch or segment of the nanofabric suspended between twoother traces disposed in crossed-relation to the patch or segment.

Referring to FIG. 1A, a silicon wafer substrate 100 with an insulatingor oxide layer 102 is provided. (Alternatively, the substrate may bemade from any material suitable for use with lithographic etching andelectronics, and the oxide layer can be any suitable insulator.) Theoxide layer 102 has a top surface 104. The oxide layer 102 is preferablya few nanometers in thickness, but could be as much as 1 μm thick. Theoxide layer 102 is patterned and etched to generate cavities 106 to formsupport structure 108.

The width W of cavity 106 depends upon the type of lithographicpatterning used. For example, with present photolithography this cavitymay be about 180 nm wide. With other approaches the width may be asnarrow as about 20 nm or smaller. The remaining oxide material definessupports 110 on either side of cavity 106.

Referring to FIG. 1B, material is deposited in the cavities 106 to forma lower electrode 112; the electrode material can be chosen from anysuitable conductor or semiconductor. The lower electrode 112 isplanarized such that its top surface is substantially level with the topsurface 104 of oxide layer 102, forming intermediate structure 114.Lower electrode 112 can be a prefabricated contact plug or a via. Also,lower electrode 112 can be deposited or fabricated in other ways,including by forming on the surface of substrate 102.

Referring to FIG. 1C, a nitride layer 116 (or any suitable insulator) isdeposited on the surface of structure 114, forming intermediatestructure 118. Nitride layer 116 has a top surface 120. A non-limitingexample of nitride thickness is approximately 20 nm for 0.18 micronground rule (GR). The nitride thickness may vary depending on the groundrule of the desired final product. As explained below, these dimensionscan affect certain characteristics of the device; for example, in thecase of a discrete memory cell the parameters may determine whether theswitch is non-volatile or volatile and may affect the V_(on) and V_(off)voltages for causing the nanofabric article to deflect.

Referring to FIG. 1D, nitride layer 116 of structure 118 is thenpatterned and etched to generate cavities 122. The size and shape of thecavity 122 is made to correspond to what will eventually become nanotubeactive regions, i.e., the regions in which nanofabric articles may becaused to deflect. The cavity 122 is formed substantially above lowerelectrode 112, and leaves remaining nitride layer 124 and formsintermediate structure 126.

Referring to FIG. 1E, a sacrificial layer 128 of polysilicon (having topsurface 131) is deposited on the surface of intermediate structure 126,forming intermediate structure 130. A non-limiting parameter for thethickness T of polysilicon layer 128 is on the order of 100 to 200 nm.

Referring to FIG. 1F, the top surface 131 of intermediate structure 130is planarized. By doing so the surface 133 of the remaining polysiliconlayer 132 is substantially level with the top surface 120 of remainingnitride layer 124, thus forming intermediate structure 134.

Referring to FIG. 1G, a nanotube fabric 136 is applied to, or formed on,the surface of intermediate structure 134, thus forming intermediatestructure 138. Non-limiting methods of applying such a fabric 136 are byspin coating pre-formed nanotubes, aerosol application, dipping or bychemical vapor deposition. Such methods are described in the referenceslisted and incorporated above.

Referring to FIG. 1H, a resist layer 140 is applied to the top surfaceof intermediate structure 138, forming intermediate structure 142.

A region of the resist layer 140 is then patterned. The region should beover the area designated for the nanotube active region and should belarger than such. The resist layer 140 may be patterned by firstlithographically patterning the resist layer 140 forming intermediatestructure 144, as shown in FIG. 1I. Structure 144 has exposed portions146 of nanofabric on either side of patterned resist 148.

Then, as shown in FIG. 1J, exposed nanotube fabric 146 may be etchedaway, forming intermediate structure 150. A non-limiting method ofetching the nanotube fabric is by plasma ashing. Structure 150 haspatterned resist 148 with similarly patterned nanofabric portion 141underneath.

Referring to FIG. 1K, the patterned resist layer 148 is removed, formingintermediate structure 152, having patterned segment or patch 154 ofnanotube fabric. The patch 154 is over region 132 of sacrificialmaterial which is over electrode material 112. The patch is slightlylonger than the width of the polysilicon region 132.

Referring to FIG. 1L, a polysilicon layer 156 is deposited over the topsurface of intermediate structure 152 to form intermediate structure158. A non-limiting range of polysilicon layer 156 thickness T isbetween about 20 to 50 nm.

Referring to FIG. 1M, the polysilicon layer 156 is patterned formingintermediate structure 162. Structure 162 has a remaining polysiliconlayer portion 160 over the patch of nanotube fabric 154, which as statedabove is positioned in what will be a nanotube active region.Polysilicon layer portion 160 is larger than what will be nanotubeactive region 122 and is the same size or larger than the underlyingpatterned nanotube fabric 154.

Referring to FIG. 1N, top electrode material 164 is deposited over thetop surface of intermediate structure 162, forming intermediatestructure 166. A non-limiting thickness T of electrode material 164 ison the order of about 350 nm. The material for use as top electrode 164can be selected from any metal or conductor suitable for electroniccomponents. Alternatively, depending on the ultimate use of the devicefabricated, this material could be an insulator, e.g., if it were to beused as a nanotube protective layer. The top electrode could also bedefined as a line or a slot landing pad or other structure suitable forinterconnection.

Referring to FIG. 1O, top electrode material 164 is patterned to formupper electrode 168. The top electrode could also be defined as a lineor a slot landing pad or other structure suitable for interconnection.FIG. 1O does not show upper encapsulation material for the sake ofclarity.

Referring to FIG. 1P, remaining polysilicon layer portion 160 andremaining polysilicon 132 are etched away to create structure 176. Theelectrodes 168 and 112 extend perpendicularly relative to the page andare supported at ends away from the nanotube active area in which thepatch 132 of nanofabric is suspended. The patch 154 is suspended withgaps, e.g., 174, defined by the thicknesses of the sacrificial materialthat was removed, e.g., 160. FIG. 1P does not show upper encapsulationmaterial for the sake of clarity. FIGS. 2A-D, however, illustrate howupper material 178 is used to encapsulate the structure and to assist inclamping the suspended nanotube fabric article.

The process described above can be modified in many ways. For example,the steps corresponding to FIGS. 1I-L may be substituted as follows.Referring to FIG. 1I′ (which would follow steps corresponding to FIG.1H), resist layer 140 (see FIG. 1H) is patterned to leave photoresist149 and have exposed nanotube portion 147, forming intermediatestructure 145.

Referring to FIG. 1J′, a layer 151 of polysilicon is deposited over whatwas the exposed nanotube region 147 (see FIG. 1I′) and onto theremaining photoresist layer 149, forming intermediate structure 153.Polysilicon layer 151 may be any material useful in CMOS processing solong as it is differently etchable over resist layer 140 and the exposednanotubes.

Then, referring to FIG. 1K′ remaining photoresist layer 149 is removedin a liftoff process, forming structure 155 with exposed nanotube fabricportions 157. The process then continues with that mentioned abovestarting with the description of FIG. 1M.

The exposed nanotube fabric portions 157 may be removed in an ashingprocess, leaving polysilicon layer 151 over nanotube fabric segment 154,forming structure 162. The remaining polysilicon portion 151 is largerthan what will be the nanotube active area, similar to the situationabove, and similar subsequent steps may be performed to complete thestructure.

FIGS. 2A-D illustrate a metallization and encapsulation scheme that canbe used with structure 176 of FIG. 1P. Specifically, the structure 176has been encased by insulating material 178. Depending on the techniquesemployed the region in which the nanofabric article is suspended may bevacuum.

The structure so formed is a tri-stable or tri-trace device. Forexample, some of the patent applications identified and incorporatedabove describe various ways in which tri-stable or tri-trace devices maybe used. Among other ways, the tri-trace device may be used as aredundant way in which to activate the suspended nanotube article; maybe used to encode tertiary information; may be used in a push-pullarrangement with the suspended article. In addition, it may be used sothat one trace is used to deflect the nanotube article into contact withan electrode, and the other trace may be used to release the nanotubearticle from contact.

The nanoswitch in structure 182 has been encased by insulating material178, and has a gap height 180. In some embodiments, the gap height 180is a function of the thickness of sacrificial polysilicon layers 132,160 (see FIG. 1(O) above). Upon deflection, the nanofabric contacts thelower electrode 112 forming a stable van der Waals interaction yieldinga non-volatile switch.

Structure 183 illustrates a nanofabric based switch having an insulationlayer 185 over one electrode. (Fabrication of such an oxidized electrodeis explained below in Example 3.) The insulation layer 185 may be usedto change the characteristics of the switch to be volatile or to providefurther assurance of desired behavior. The insulating layer (whichalternatively may have been placed on the facing surface of electrode168) may be used to prevent different fibers from the nanofabric elementfrom simultaneously electrically contacting both electrodes (112, 168)during a state transition. Such contact may prevent or hinder switchingof the fabric between states.

Compare structures 182 and 183, which may be used as non-volatileswitches, to structure 188 which illustrates a volatile switch. Instructure 188 the gap height 186 between the nanofabric 172 and theunderlying electrode 112 has been increased such that the strain energyof the stretched nanofabric overcomes the van der Waals attractionbetween the fabric and the electrode. The nanofabric forms part of aclosed circuit and returns to its non-deflected, open circuit state. Itshould be noted that the effect of the van der Waals interaction betweennanofabrics and other elements can be affected at their interface(s).The effect may be enhanced or diminished; e.g., the attractive force canbe diminished by coating the surface of the electrode with a thin layerof oxide or other suitable materials. A purpose of this diminishing ofattractive forces may be to create volatile nanoswitches; such volatileswitches may be especially useful in applications such as relays,sensors, transistors, etc.

In the embodiment of FIG. 2A, the electrode or electrodes may beactivated relative to the patch 154 to cause the patch 154 to deflectand contact the lower electrode 112. In this case, this forms a stablevan der Waals interaction. The deflection of the patch also creates arestoring force to restore the patch to horizontal (non-deflected) stateshown in FIG. 2A. This restoring force is a function of, among otherthings, the geometry of the device, e.g., the distance (180) which thepatch 154 is deflected. In this embodiment, the van der Waals forcewhich keeps the patch 154 in contact with the electrode 112 is largerthan the restoring force resulting from the geometry of FIG. 21,yielding a non-volatile switch. That is, when power is removed, the vander Waals force that holds the patch 154 in contact with electrode 112is greater than the restoring force on the patch 112, and thus the patchwould remain in a deflected state.

Compare this situation to the structure 188 of FIG. 2C. In FIG. 2C, thegap distance 186 is larger creating a larger restoring force. Byappropriate control of the gap distances (via creation of thesacrificial material) this gap may be made large enough to create arestoring force that is greater than the van der Waals force.Consequently, the device of FIG. 2C would be volatile. The patch 154could be deflected similarly to that described above, but the restoringforce would be large enough to cause the patch 154 to return to thehorizontal state if power were interrupted. The nanoribbon can deflectvia electrostatic attraction but the van der waals force by itself isnot sufficient to hold it there. In structure 188 the gap height 186between the patch 154 and the underlying electrode 112 has beenincreased such that the strain energy of the stretched nanofabricovercomes the van der Waals attraction between the fabric and theelectrode. The nanofabric forms part of a closed circuit and returns toits non-deflected, open circuit state.

FIG. 2D illustrates structure 192. Structure 192 illustrates anon-volatile switch where deflected nanofabric 154 contacts lowerelectrode 112 closing a circuit and remains in contact keeping thecircuit closed indefinitely. If the gap height 180 of structure 192 weresufficiently large as in structure 188 then the deflected state of thenanofabric 172 would not remain indefinitely.

By properly supporting nanofabric 154, the amount of deformation of thenanofabric 154 can be affected. For example, as shown in FIG. 2(A), thenanofabric 154 may be “pinched” at the edges of the open region 194 leftafter removal of sacrificial polysilicon layer 132, previously shown inFIG. 1(O). Having upper and lower pinching support around nanofabric 154can increase the strain on the nanofabric 154. In some embodiments, thistype of pinching support at the edges of the open region 194 createsvolatile switches which would otherwise be non-volatile. By controllingthe design and manufacture of the nano-switch assembly as describedhere, it is possible to selectably provide tri-state non-volatilestructures and/or volatile structures.

Using discrete nanofabrics articles and electrodes in this fashionpermits formation of discrete devices and cells. For example, thesecells may be used in digital memory devices. The nanofabrics, e.g. 154,and electrodes, e.g. 168, may extend above the substrate and/or supports102 sufficiently to allow an electrical connection to be made to thenanofabrics 154 and electrodes 168. Such connections may be made by anysuitable method, such as by etching or exposure to form a channel 196(not to scale) or via connecting the nanofabrics 154 with an activationelectrical signal.

Channel 196 is used for electrically connecting an element of theswitch, e.g. nanofabric 154, to an activation (read/write) line. Thechannel 196 may subsequently be filled with a conductor to achieve theactivation connection, or may be formed by some other technique.

One aspect of the present invention is directed to formation ofconductive composite junctions whereby a suitable matrix material isdisposed within and around the nanotubes or fibers of a nanofabric orother porous nano material. Such junctions can provide desirablemechanical and/or electrical properties. For example, electrical contactbetween a nanofabric and a metal connection or activation point may beenhanced, or the contact resistance may be decreased by applying themetal contact as a matrix material impregnating the nanofabric tubes.Also, mechanical contact and strain may be increased as a result of theincreased contact between the nanotubes and the matrix material.

For example, with reference to FIG. 2D, activation connection channel196 extends down to the nanofabric 154. Then a metal filling the channel196 may further be introduced into the pores of nanofabric 154, inregion 194. The matrix material extends down to the underlying nitridelayer (or any other layer) below the nanofabric 154. The effect of sucha connection can be to secure the nanofabric 154 and increase the strainon the nanofabric 154. Also, the electrical connection betweennanofabric 154 and the activation connection is increased. Other methodsof securing the nanofabric to the supports are envisioned and one isexplained in Example 2, below.

Almost any material (insulating or conducting) can be made to penetrateinto or through a porous thin article such as a nanofabric. This may bedone to improve the mechanical pinning contact and increase reliabilityand manufacturability, or to improve electrical connection with thenanofabric and reduce contact resistance to the nanoarticle. Dependingon the materials used, a bond may form between the penetrating matrixmaterial and the material below the nanofabric. Examples of materialswhich can be used to secure a nanofabric in this way include metals andepitaxial silicon crystal materials. Other uses for such junctions arepossible, for example in the manufacture of permeable base transistors.It is worth noting that the composite junctions and connectionsdescribed above do not cause a disruption in the fabric of thenanofabric materials into which the impregnating matrix material isintroduced. That is, connection channel 196 does not itself cut throughthe nanofabric 154, but rather just allows a filler matrix material toflow into and through the nanofabric 154 and connect it to othercomponents of the device. In some cases it may be desirable to use aconductive filler to reduce resistance of the nanofabric or contacts tothe nanofabric.

FIG. 3 illustrates a view of intermediate structure 176 (see FIG. 1P)directly from above. An oxide layer 102 supports a nanofabric 154 andnitride layers 116 support electrode 168. Cross sections A-A′, B-B′ andC-C′ are shown for reference. Upper encapsulation material 178 (see FIG.2A) is omitted for clarity.

FIG. 4 is a perspective view of intermediate structure 176 taken atcross section A-A′ shown in FIG. 3. FIGS. 5 and 6 perspective views ofintermediate structure 182 taken at cross sections B-B′ and C-C′(structure 176 is structure 182 with the top insulating layers removedfor clarity). (In FIGS. 4-6 upper material 178 is shown.)

FIG. 4 is an illustration of the elements of the device of FIG. 3, asviewed along cross section at A-A′. (Again encapsulating material 178 isremoved from the figure for clarity.). At this portion of the structure,which is away from the nanotube active region, the upper electrode 168is disposed on top of nitride layer 116.

FIGS. 5(A)-(B) illustrate two views of the structure as viewed alongcross-section B-B′. In these instances the structure 182 includes theencapsulating material 178 and corresponds to the view of FIGS. 2A-B.FIG. 5A shows the view along cross section B-B′, and FIG. 5B shows theview along cross-section B-B′ and again in cross section along C-C′.

These views show the patch 154 suspended in an active region between anupper electrode 168 and lower electrode 112. As stated above, andexplained in the identified and incorporated patent applications, theelectrodes may be used to cause the patch 154 to deflect up or down. Thepatch 154 is clamped by material from above and below to the edge of thenanotube active region, where the patch is suspended 154 and may becaused to deflect as shown in FIG. 2C. In this embodiment, thedisplacement D of FIG. 2C has been substantially removed. A substratelayer 100 supports an oxide layer 102. A lower electrode 112 is disposedbelow and not in contact with nanofabric 154 which is fixed toinsulating layer 178 and insulating layer 116 supports electrodematerial 168. For the sake of clarity, the patch 154 is not illustratedwith an electrical contact, but metalization techniques such as thosedescribed in connection with FIG. 2C may be employed. As stated above,the gaps between the patch 154 and corresponding electrode may becontrolled to create either volatile or non-volatile states.

FIG. 6 illustrates the elements of structure 182 as viewed along crosssection C-C′. The patch 154 in this cross section does not appear to becontacting any other element, but as can be seen in FIGS. 5A-B, thepatch is contacting and clamped by other elements, e.g. insulating layer178 (not shown in FIG. 6). The exploded view (shown within the dottedlines) illustrates the interrelations of substrate 100, insulating layer102, nitride layer 116 and electrodes 112 and 168, as well as thelocation of patch 158 in reference to the aforementioned elements.

The structures depicted above may be used as nano-electromechanicalswitches and can be created to have a volatile or nonvolatile state (asmanifested by the deflected state of patch 154) depending on the aspectratio of the lengths a and b, were a is the distance between theundeflected nanofabric and the electrode (i.e., the gaps 180 or 186 ofFIGS. 2A-B), and b is the length of the nanofabric which deflects. Ifthe strain energy of the deflected nanofabric is less than the van derWaals force holding the nanofabric in contact with the lower electrode,then the switch will be non-volatile. If however the strain energy canovercome the van der Waals attraction, then the switch will behave in avolatile manner and a circuit will be closed only fleetingly.

Furthermore, the switch may be volatile with regard to top electrode 168and non-volatile with regard to lower electrode 112, or both volatile orboth non-volatile.

FIG. 7 is an actual micrograph of a working nano-fabric-based switch.The fabrication of the switch is described in Example 2, below. In thismicrograph, only a few nanotubes of a given patch appear in focus butcan be seen spanning the channel formed. (The device omits an upperencapsulating material 178, as shown in FIG. 2A.)

Example 1

To fabricate the nonvolatile nanotube switch a silicon wafer with athermally grown oxide (0.5 μm) is employed.

Embedded electrodes are constructed by electron beam lithography (EBL)with polymethylmethacrylate (PMMA) as resist. After the electrodepattern is defined in the resist, Reactive Ion Etching (RIE) is employedwith CHF₃ gas to construct a trench in the oxide. The embeddedelectrodes are constructed by a lift-off process by depositing theelectrode in an electron beam evaporator to fill the trench and thenstripping of the resist in N-methyl pyrolidone at 70° C. (Shipley 1165).The electrodes are 0.18±0.02 μm wide and consist of 850 Å of metal(Titanium, Ti) and 200 Å of a sacrificial material (Aluminum oxide,Al₂O₃). The vertical gap between electrodes and the as yet depositedSWNTs was adjusted at 200±50 Å to yield electromechanically switchablebits as predicted theoretically. This gap of 200±50 Å corresponds to atensile strain of ε_(tensile)=0.9±0.5% of the nanotubes in the ON state,which lies well within the elastic limit of SWNTs. A silicon or metalbeam, however, could not withstand this tensile strain withoutpermanent, plastic deformation.

After the creation of the embedded electrodes, the carbon nanotubefabric is constructed. The nanotube fabric is produced by spin-coating asolution of SWNTs in 1,2 dichlorobenze (ortho-dichlorobenzene, ODCB) onthe device wafer. The concentration of the nanotube solution is 30±3mg/L. The solution is sonicated in an ultra-sonic bath (70 W sonicationpower) for 90 minutes to fully disperse the nanotubes. After sonication,the nanotube solution is spun onto the wafer utilizing typicalphotoresist spinning techniques. Multiple spins were required to producethe desired sheet resistance of the SWNT fabric of <100 kΩ/square. Thesheet resistance of the nanotube monolayer can be reliably variedbetween 10 Ωs/square and several MΩ/square by adjusting theconcentration of the nanotube solution and the number of spin-coatingsteps. For the devices discussed here, a SWNT sheet resistance of 75kΩ/square was chosen.

Once the desired nanotube fabric sheet resistance and density isobtained, positive i-line photoresist is spin-coated on the SWNTs (e.g.Shipley 1805 resist). The patterning of the nanotubes, however, is notlimited by the type of photoresist employed, since various types ofresist have been used. The photoresist-coated wafer is then exposed anddeveloped to form the desired pattern. After development of the pattern,the exposed carbon nanotubes can be removed cleanly by isotropic ashingin oxygen plasma while the nanotubes underneath the photoresist areprotected from oxidation. Typically an isotropic oxygen plasma of 300 Wpower was used for removing the exposed SWNTs at a pressure of 270 mtorrand an ashing time of 9 minutes. The photoresist is then subsequentlystripped in N-methyl pyrollidinone (NMP) and a SWNT film pattern isexposed. Patterned SWNT stripes were typically 100 μm long and 3 μmwide, although stripes with widths as small 0.25 μm have also beenfabricated.

In a subsequent, aligned EBL step, clamp lines (Ti, 1000 Å thick,0.18±0.02 μm wide) are defined by liftoff in PMMA resist on top of theSWNT stripes, parallel to the embedded electrodes (distance of 1000 Å toelectrode). These clamps are necessary to prevent uncontrolled adhesionof the SWNTs to the lower electrode upon removal of the sacrificiallayer in the next step. Subsequently, the device electrodes and SWNTstripes were interconnected to bond pads so that individual junctions ona die could be electrically tested. The distance between SWNT stripemetallization and switching junction is 3 μm. Finally, the patternedSWNTs were suspended by wet chemical removal of the Al₂O₃ sacrificiallayer in an aqueous base (ammonium hydroxide, NH₄OH), followed by rinsein deionized water (DI) and isopropanol (IPA). Subsequently, the devicedie were hermetically packaged.

Programmable nanotube memory devices fabricated according to thisprocedure were programmed by sweeping the voltage over the junctionusing the programmable voltage source of a Keithley Electrometer.Simultaneously, the current that flew over the junction was measuredusing the integrated current preamplifier (10 fA sensitivity) of theelectrometer to generate current vs. voltage curves (I-V curves). Forall measurements the SWNTs were biased high, while the underlyingelectrode was held at ground. Current vs. voltage (I-V) measurementsshowed an abrupt increase in current over the nanotube-electrodejunctions at a threshold voltage of 2.5±0.5 V as the SWNTs switched froma suspended, high resistance (>MΩ) OFF state into contact with theunderlying electrodes to form an ohmic (˜kΩ) ON state. The nanotube bitstate was retained even when power was disconnected for several days(i.e., the switched bits are nonvolatile).

Example 2

A wafer (32-04, Die E4, Device 9×26/4×17) is coated in resist andpatterned with standard optical lithographic technique, the pattern wastransferred to the SiO₂ by reactive ion etch (RIE) in CHF₃ for 4minutes.

A Cr/Au 5/50 nm marker was used for EBL alignment (via thermalevaporation). The resist and the metal above the resist were removed bya standard liftoff in N—N Dimethyl pyrolidone (1165). The wafer wasashed in O₂ plasma for 5 min PMMA. (Microchem), was applied by spincoating 60 seconds at 4000 rpm.

Electron bean lithography (EBL) was performed to make EBL markers, PMMAwas developed and the pattern was etched into the SiO2 in CHF₃ for 4min. 5/50 nm Cr/Au was deposited, and a liftoff was performed as aboveto leave the EBL markers. PMMA was applied, and EBL was performed tocreate the lower electrode pattern. The PMMA was developed with MIBK.RIE was performed in CHF3 for 4:30 minutes to transfer the pattern tothe SiO₂.

85/20 nm Ti (conductor)/Al₂O₃ (sacrificial layer) was Electron Beamdeposited, and lifted off as described above to create the lowerelectrodes.

Atomic Force Microscopy (AFM) was performed to determine theunder/overfill of the lower electrode, the electrodes were underfilledby −2 nm).

Laser ablation-grown nanotubes in solution were spun on to the wafer, 8times to produce a film with a 50 Kilo Ohm resistance (500 rpm for 30sec. and 2000 rpm for 20 seconds, 8 times).

Photoresist was spun onto the nanotube fabric (1 Min 400 rpm). Theresist was patterned and developed and the wafer was ashed in an O₂plasma for 3 min at 300 W three times, with 5 minute intervals to removethe exposed nanotubes. The remaining resist was removed in 1165(Shipley). PMMA was applied as above, and EB lithography was performedto create a pattern of clamps which attach the nanotube fabric moresecurely to the underlying supports, (100 nm Ti). Interconnects (notshown in the micrograph) are created by first applying and developingresist as above, and upper interconnect metal material (10/10/250/10Cr/Au/Cu/Au) was deposited and a lift off procedure was performed. Thesacrificial Al₂O₃ layer was removed by wet etch in 4:1 Deionizedwater:351 (Shipley) (an NaOH based developer). The junction shown inFIG. 7 was created by the method outlined in Example 2. The lightvertical stripes are raised supports; the single dark stripe is a lowerelectrode below suspended nanotubes. Because of the resolution of theelectron microscope, the image does not clearly show the nanotubes, itdoes, however show the geometry of the switch.

Example 3

A junction created as described in Example 2 was oxidized in order toincrease the reliably volatile aspect of switches as follows:

Five standard cubic centimeters per minute (sccm) of O₂ was flowed overan NRAM switch, ac voltage (triangle wave) was applied to the NRAMjunction (5 V amplitude, 10 kHz frequency).

Amplitudes lower than 2 V are not high enough to make the switchvolatile. Amplitudes higher than 7 V frequently destroy the device (veryhigh to infinite resistance afterwards). It was found that the switchturns volatile within a few seconds of application of voltage in thepresence of the O₂, after which, the switch remained volatile. 5Vamplitude of ac wave adequately oxidizes the electrode; however voltageamplitudes of 2 V-7 V have been successfully used for fabricatingvolatile devices.

FIGS. 8A-9 illustrate various ways in which a nanotube fabric may beclamped or pinched or strapped by various materials including metalcoverings. This may be done to better hold the nanotube patch and toprovide low resistance interconnect to the patch.

FIG. 8(A) illustrates a framed portion of nanofabric and a method of itscreation. Such a framed nanofabric may be created by first creating afabric 802 on a substrate, as illustrated by intermediate structure 800,covering the fabric 802 with an appropriate covering material 812, asshown illustrated by intermediate structure 810, and patterning andremoving a section of the covering material 812, e.g. by lithographyleaving a “frame” of material around exposed fabric, as shown inintermediate structure 814. Such a strapping method is more fullydescribed in U.S. Provisional Patent Application No. 60/476,976, filedJun. 9, 2003, entitled Non-volatile Electromechanical Field EffectTransistors and Methods of Forming Same. The covering material may beconductive, and may act to alter the electrical properties of the entirepatterned fabric, or it may be semiconducting or insulating. Thematerial of the strapping layer should be selectively etchable overnanofabric when used alone to open up a window of exposed fabric. Thematerial of the covering layer may be selectively etchable over anintermediate layer disposed between the nanofabric and covering layer.The intermediate layer in this case may act as an etch stop when etchingand patterning the covering layer.

FIG. 8(B) illustrates a patterned fabric where no frame is formed,rather a set of disconnected sections of covering layer are formed,disconnected sections may be electrodes and have particularly usefulapplication for resistance modulation detection structures. Intermediatestructure 810 is patterned to form electrodes 818, as illustrated inintermediate structure 816.

FIG. 9, intermediate structure 900, illustrates yet another method offorming nanofabric-based devices. Such a method involves a coveringmaterial 902 that is selectively etchable over an intermediate layer904. Covering material 902 is preferably metal, and intermediate layeris preferably a semiconductor, e.g. silicon, however any suitablematerial for the application will work. The intermediate layer 904 isdisposed between the nanofabric 906 and covering layer 902. Theintermediate layer 904 in this case may act as an etch stop when dryetching and patterning the covering layer 902. Intermediate structure910 illustrates patterned covering layer 912 in the shape of a frame,however any pattern will work depending on the requirements of the finalproduct. Intermediate structure 910 is subjected to an annealing step(forming structure 916) whereby covering layer 902 and intermediatelayer form a conducting composite layer 914, e.g. a metal silicide. Sucha composite layer can act as stitching electrode or other contact oraddressing element, depending on the use of the final product.

FIG. 10 is an image of an exemplary fabric of nanotubes shown inperspective. As can be seen, the fabric may be highly porous and appearas several threads with gaps in between. In this figure there areactually several ribbons of nanofabric extending from left to rightseparated from one another by areas with no nanotubes. One may noticethat the fabric of FIG. 7 is likewise very porous with a few nanotubesspanning the channel and contacting electrodes. In both figures, theresolution of the figure is affected by the imaging technology so somenanotubes may not appear in focus or be noticeable.

Other Variations

Note that the electrodes, e.g. the top electrode 168, may themselves beformed of nanofabric materials. In some embodiments, having a nanofabricribbon or other nanofabric article disposed above the movable nanofabricelement 172 instead of a metallic electrode permits removal ofsacrificial materials from below the top electrode. Fluid may flowthrough a nanofabric material disposed above a sacrificial layer toremove the sacrificial material. Likewise, the lower electrode may beformed of a nanofabric material if desired.

Under certain preferred embodiments as shown in FIGS. 2(A)-(B), ananotube patch 154 has a width of about 180 nm and is strapped, clamped,or pinned to a support 102 preferably fabricated of silicon nitride. Thelocal area of lower electrode 112 under patch 154 forms an n-dopedsilicon electrode and is positioned close to the supports 110 andpreferably is no wider than the patch, e.g., 180 nm. The relativeseparation from the top of the support 102 to the deflected positionwhere the patch 154 attaches to electrode 112 (FIG. 2(B)) should beapproximately 5-50 nm. The magnitude of the separation (180 or 186) isdesigned to be compatible with electromechanical switching capabilitiesof the memory device. For this embodiment, the 5-50 nm separation ispreferred for certain embodiments utilizing patch 154 made from carbonnanotubes, but other separations may be preferable for other materials.This magnitude arises from the interplay between strain energy andadhesion energy of the deflected nanotubes. These feature sizes aresuggested in view of modern manufacturing techniques. Other embodimentsmay be made with much smaller (or larger) sizes to reflect themanufacturing equipment's capabilities.

The nanotube patch 154 of certain embodiments is formed from a non-wovenfabric of entangled or matted nanotubes (more below). The switchingparameters of the ribbon resemble those of individual nanotubes. Thus,the predicted switching times and voltages of the ribbon shouldapproximate the same times and voltages of nanotubes. Unlike the priorart which relies on directed growth or chemical self-assembly ofindividual nanotubes, preferred embodiments of the present inventionutilize fabrication techniques involving thin films and lithography.This method of fabrication lends itself to generation over largesurfaces especially wafers of at least six inches. The ribbons shouldexhibit improved fault tolerances over individual nanotubes, byproviding redundancy of conduction pathways contained with the ribbons.(If an individual nanotube breaks other tubes within the rib provideconductive paths, whereas if a sole nanotube were used the cell would befaulty.)

While the inventors typically desire a monolayer fabric of single-wallednanotubes, for certain applications it may be desirable to havemultilayer fabrics to increase current density, redundancy or othermechanical or electrical characteristics. Additionally it may bedesirable to use either a monolayer fabric or a multilayer fabriccomprising MWNTs for certain applications or a mixture of single-walledand multi-walled nanotubes. The previous methods illustrate that controlover catalyst type, catalyst distribution, surface derivitization,temperature, feedstock gas types, feedstock gas pressures and volumes,reaction time and other conditions allow growth of fabrics ofsingle-walled, multi-walled or mixed single- and multi-walled nanotubefabrics that are at the least monolayers in nature but could be thickeras desired with measurable electrical characteristics.

The effect of the van der Waals interaction between nanofabrics andother elements can be affected at their interface(s). The effect may beenhanced or diminished; for example, the attractive force can bediminished by coating the surface of the electrode with a thin layer ofoxide or other suitable chemicals. Volatile nanoswitches may also bemade by employing such techniques instead of or in addition tocontrolling the gap dimension between a patch and electrode. Suchvolatile switches may be especially useful in applications such asrelays, sensors, transistors, etc.

As the vertical separation between the patch and the underlyingelectrode increases, the switch becomes volatile when the deflectednanofabric has a strain energy greater than that of the van der Waalsforce keeping the fabric in contact with the underlying electrode. Thethicknesses of insulating layers which control this separation can beadjusted to generate either a non-volatile or volatile condition for agiven vertical gap as called for by particular applications with desiredelectrical characteristics.

Other embodiments involve controlled composition of carbon nanotubefabrics. Specifically, methods may be employed to control the relativeamount of metallic and semiconducting nanotubes in the nanofabric. Inthis fashion, the nanofabric may be made to have a higher or lowerpercentage of metallic nanotubes relative to semiconducting nanotubes.Correspondingly, other properties of the nanofabric (e.g., resistance)will change. The control may be accomplished by direct growth, removalof undesired species, or application of purified nanotubes. Numerousways have been described, e.g. in the incorporated references, supra,for growing and manufacturing nanofabric articles and materials.

The U.S. Patent Applications, identified and incorporated above,describe several (but not limiting) uses of nanofabrics and articlesmade therefrom. They also describe various ways of making suchnanofabrics and devices. For the sake of brevity, various aspectsdisclosed in these incorporated references are not repeated here. Forexample, the various masking and patterning techniques for selectivelyremoving portions of the fabric are described in these applications; inaddition, various ways of growing nanofabrics or of forming nanofabricswith preformed nanotubes are described in these applications.

As explained in the incorporated references, a nanofabric may be formedor grown over defined regions of sacrificial material and over definedsupport regions. The sacrificial material may be subsequently removed,yielding suspended articles of nanofabric. See, for example,Electromechanical Memory Array Using Nanotube Ribbons and Method forMaking Same (U.S. patent application Ser. No. 09/915,093, now U.S. Pat.No. 6,919,592), filed Jul. 25, 2001, for an architecture which suspendsribbons of nanofabric.

The articles formed by preferred embodiments help enable the generationof nanoelectronic devices and may also be used to assist in increasingthe efficiency and performance of current electronic devices using ahybrid approach (e.g., using nanoribbon memory cells in conjunction withsemiconductor addressing and processing circuitry).

It will be further appreciated that the scope of the present inventionis not limited to the above-described embodiments but rather is definedby the appended claims, and that these claims will encompassmodifications and improvements to what has been described.

1. A device, comprising: a structure defining a gap having a gap width;a defined segment of nanotube fabric, disposed on the structure andspanning the gap, the nanotube fabric segment including a plurality ofnanotubes, at least some of said nanotubes having a length in excess ofthe gap width; and a clamp, disposed at each of two ends of the nanotubefabric segment and disposed over at least a portion of the nanotubefabric segment substantially at the edges defining the gap.
 2. Thedevice of claim 1, wherein the clamp is made of electrically-conductivematerial.
 3. The device of claim 1, wherein the clamp is made ofelectrically-insulative material having a via therethrough filled withelectrically conductive material to provide an electrical communicationpath with the nanotube fabric segment.
 4. The device of claim 3 whereinthe via is filled with metal to provide a metallic signal path to thenanotube fabric segment.
 5. The device of claim 3 wherein the nanotubefabric segment is made of a nanofabric having a porosity and wherein theelectrically conductive material filling the via also fills at leastsome of the pores of the of the nanotube fabric segment.
 6. The deviceof claim 1, wherein the nanotube fabric segment has alithographically-defined shape.
 7. The device of claim 6, furthercomprising an electrically-conductive trace disposed in the gap andbeing in spaced relation with the nanotube fabric segment.
 8. The deviceof claim 1 wherein the clamp is a structure defining a second gap abovethe nanotube fabric segment and wherein the gap and the second gap eachhave respectively a conductive trace disposed therein.
 9. The device ofclaim 1 wherein the nanotube fabric segment is made of a nanofabrichaving a porosity and wherein the clamp is made of material that fillsat least some of the pores of the of the nanotube fabric segment. 10.The device of claim 1 further including at least one electricallyconductive trace in spaced relation relative to the nanotube fabricsegment and wherein the nanotube fabric segment is electromechanicallydeflectable into contact with trace in response to electricalstimulation of the trace and nanotube fabric segment.
 11. The device ofclaim 10 wherein the contact with the trace is a non-volatile state. 12.The device of claim 10 wherein the contact with the trace is a volatilestate.
 13. The device of claim 10 wherein the at least one electricallyconductive trace has an interface material to alter the attractive forcebetween the nanotube fabric segment and the electrically conductivetrace.